This invention relates to the amplitude modulation of an injection laser with binary digital information, such modulation being produced by modulation of the drive current applied to the laser. This drive current may be considered as composed of a bias current and a modulation current, the bias current being the mean of the data 1 bit current and the data 0 bit current, and the modulation current being the current that is superimposed upon the bias current, augmenting it during data 1 bit periods to the data 1 bit value, and diminishing it during the data 0 bit period to the data 0 bit value.
Especially in the case of modulation at high bit rates, it is preferred to arrange for the drive current in data 0 bit periods to be a non-zero current not much greater than the value corresponding to the lasing threshold current. A lower drive current is undesirable because of the time delay and frequency chirp associated with attempting to drive an injection laser rapidly hard on from well beneath lasing threshold. On the other hand a significantly higher drive current would produce excessive amounts of light during such periods, thereby significantly impairing the contrast ratio between the data 1 and data 0 bits achieved with a particular value of bias current.
Having regard to the fact that the lasing threshold of an injection laser varies with temperature and with the effects of ageing, and that similar properties are also observed in respect of slope efficiency (ratio of incremental laser light output to current drive), a conventional laser driver incorporates some form of monitor photodetector, typically a photodiode, positioned to receive a proportion of the emitted laser radiation, typically that emitted from the rear facet of the laser, and the output of this monitor photodetector is then used for feedback purposes regulating the current drive.
In one method of regulation for balanced code binary modulation, a low frequency is superimposed on the amplitude of data 0 bits. The mean output of the photodetector is used to control the bias current, and then the modulation current is adjusted to provide a specific amplitude of low frequency ripple in the photodetector output. The value of this ripple in the photodetector output changes relatively abruptly in the vicinity of the lasing threshold because the slope efficiency below lasing threshold is relatively low and changes rapidly near threshold to a significantly higher value above threshold.
One drawback of this use of a ripple frequency to control the operation of the driver is that the binary data stream may, from time to time, include a spectral component at the ripple frequency. This can give rise to spurious effects if this spectral component is detected by the feedback control loop and falsely interpreted as an indication that the current drive is too great in the data 0 bit periods. Such a difficulty can sometimes be avoided by ensuring that the bit stream is such that it can not contain such a frequency component, or by using phase sensitive techniques and ensuring that the ripple is always in phase quadrature with any spectral component of the ripple frequency present in the data stream. Even in these circumstances there is still the drawback that the feedback technique has involved putting an additional frequency component upon the optical output, and this can cause problems elsewhere in the transmission system. Additionally it will be noted that the speed of response is limited by the low frequency of the imposed ripple.
An alternative class of methods of regulation in effect rely on peak detection, such as for instance is described in GB 1539624. In essence the basic peak detection method involves measuring the amplitude of peaks in photodetector output, and then adjusting the modulation depth to ensure that these peaks are not quite twice the magnitude of the mean value of the photodetector output. In this way it is ensured that the current drive of data 0 bits is not quite beneath the lasing threshold value.
A drawback of this peak detection method, particularly for high bandwidth data transmission systems, is that the photodetector output needs to be amplified by quite a large amount to provide a signal of sufficient amplitude to operate a typical peak detector. This means that the amplifier needs to be both high bandwidth and high gain, and a further disadvantage is that the system is responsive to the amplified noise contaminating the data 1 bits.